I've never had one that sounds to have been as bad as some of the ones written up on here. I've relied on either yelping/"no bite" or gently holding the muzzle/"no bite"...and the phase passed with all of my fingers intact.
I'm not sure how I'd deal with one that seemed to be "fierce" in addition to mouthy (which yours may or may not be but one recent one was).
I never had success with providing a toy...my fingers were the toy that the individual pup was focused on. The substitute was always ignored.